1. Field of the Invention
The present invention relates to an ignition coil, which is used to generate a spark from a spark plug of an internal combustion engine, and a manufacturing method and a manufacturing apparatus for manufacturing such an ignition coil.
2. Description of Related Art
An ignition coil, which is used to generate a spark from a spark plug in an internal combustion engine of, for example, a vehicle, includes a primary coil and a secondary coil arranged in a coil case. Furthermore, a central core, which is made of a magnetic steel plate(s), is arranged radially inward of the primary coil and the secondary coil. A dielectric sheet, which serves as a thermal stress relief member, is wound around an outer peripheral surface of the central core. The dielectric sheet protects the dielectric resin, which is filled in spaces in the coil case, from thermal stress that is applied by the heating and cooling cycle of the engine.
The above described type of ignition coil, which includes the dielectric sheet wound around the central core, is disclosed in, for example, Japanese Unexamined Patent Publication No. H10-92670, Japanese Unexamined Patent Publication No. 2004-14548 (corresponding to U.S. Pat. No. 6,980,073) and Japanese Unexamined Patent Publication Number 2004-111714.
In Japanese Unexamined Patent Publication No. H10-92670, the dielectric sheet is interposed between a central iron core (i.e., the central core) and a secondary winding core (i.e., a spool around which a secondary coil is wound). Here, even if an air bubble is generated in the secondary winding core, which is made of thermoplastic resin, the electric insulation between the central iron core and the secondary coil is maintained by the dielectric sheet.
In Japanese Unexamined Patent Publication No. 2004-14548, the thickness of the dielectric sheet, which is wound around the central core and serves as the thermal stress relief member, is set to an appropriate thickness. In this way, formation of a crack in the dielectric resin, which is filled in the space between the central core and the cylindrical spool, is limited.
In Japanese Unexamined Patent publication No. 2004-111714, a cover member (gel) is applied to an entire length of a winding end of the dielectric sheet, which is wound around the outer peripheral surface of the central core and serves as the thermal stress relief member, so that unintentional removal and radially outward protrusion of the winding end of the dielectric sheet from the central core are limited. When the winding end of the dielectric sheet projects from the central core, stress concentration occurs in the winding end of the dielectric sheet, and a crack is generated due to thermal stress in the dielectric resin filled in the space between the central core and the coil. However, the unintentional projection of the winding end of the dielectric sheet from the central core is advantageously limited by the cover member.
Furthermore, in some previously propose cases, as shown in FIGS. 28-30, a dielectric sheet 94 is wound around a central core 923 in such a manner that an axial end portion 941 of the dielectric sheet 94 projects from an axial end surface 924 of the central core 923 to limit generation of a crack in dielectric resin 911, which contacts the axial end surface 924 of the central core 923.
However, when the dielectric sheet 94 is wound multiple times around the central core 923, axial end portions 941 of layered dielectric sheet constituent sections 940 of the dielectric sheet 94 may possibly be displaced from one another in the axial direction, so that steps 942 are formed at the axial end portions 941 of the layered dielectric sheet constituent sections 940 of the dielectric sheet 94. In such a case, as shown in FIG. 30, edged grooves (sharp grooves) 912 are formed in the dielectric resin 911 that is filled in the coil case in contact with the axial end portions 941 of the layered dielectric sheet constituent sections 940, which form the steps 942. The stress concentration may occur at the edged grooves (the sharp grooves) 912. The edged grooves 912 may possibly become an origin of the crack in the dielectric resin 911.
Furthermore, it is difficult to align the axial end portions 941 of the layered dielectric sheet constituent sections 940 in the axial direction. In view of the above disadvantage, as shown in FIG. 31, it is conceivable to make a projecting length of each axial end portion 941 of the dielectric sheet 94, which projects from the axial end surface 924 of the central core 923, longer than a required length to provide an excess region. After the winding of the dielectric sheet 94 around the central core 923, it is conceivable to mechanically cut the excess region by a cutting blade.
However, when the excess region of the dielectric sheet 94 is mechanically cut by the cutting blade, the axial end portions 941 of the layered dielectric sheet constituent sections 940 at the cutting end may possibly be spread radially outward and thereby separated from one another, as shown in FIG. 31. Furthermore, as shown in FIG. 32, corners 943 of the cut end parts of the axial end portions 941 may possibly form sharp edges. As a result, edged grooves 912, which correspond to the corners 943 having the sharp edges, are formed in the dielectric resin 911 filled in the coil case. Each of the edged grooves 912 may possibly serve as an origin of a crack in the dielectric resin 911.
Furthermore, as shown in FIG. 33, the above described disadvantage of the mechanically cutting the excess region of the dielectric sheet 94 with the cutting blade may also occur in a case of cutting the dielectric sheet 94, which is wound only once around the central core (see the corner 943 having the sharp edge). The above disadvantage of the mechanically cutting the dielectric sheet 94 with the cutting blade may also occur in a case of cutting the dielectric sheet 94 along the axial end surface 924 of the central core 923, as shown in FIG. 34.
Furthermore, as shown in FIGS. 35 and 36, in some previously proposed ignition coils, an axial end portion 951 of a sheet 95 is folded against an axial end surface 925 of a central core 923 to limit generation of a crack in dielectric resin filled in spaces in a coil case. At the time of folding the axial end portion 951 of the sheet 95, a punch 97 is used to press and weld the axial end portion 951 of the sheet 95.
However, as shown in FIG. 37, when the axial end portion 951 of the sheet 95 is pressed by the punch 97, buckling or radially outward partial deformation of the axial end portion 951 of the sheet 95 may possibly occur. This occurs due to a rigidity of the sheet 95, which limits smooth folding movement of the axial end portion 951 of the sheet 95 toward a center of the central core 923. When the buckling or deformation of the sheet 95 occurs, a crack may possibly be generated in the dielectric resin filled in the spaces in the coil case.
Particularly, when a thermoplastic resin film, such as polyethylene terephthalate (PET) film, is used to form the sheet 95, the above disadvantage may possibly occur due to a relatively high rigidity of such a film.
The present invention addresses one or more of the above disadvantages. According to one aspect of the present invention, there is provided an ignition coil, which includes a central core, primary and secondary coils, a case, a dielectric sheet and dielectric resin. The central core is produced from at least one magnetic steel plate. Primary and secondary coils are produced by concentrically winding primary and secondary electric wires, respectively, around the central core. The case receives the primary and secondary coils. The dielectric sheet is wound around an outer peripheral surface of the central core. At least one of axial end portions of the dielectric sheet includes an axially projecting portion, which projects from an axial end surface of the central core. An inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape. The dielectric resin is filled in spaces in the case.
According to another aspect of the present invention, there is provided, an ignition coil, which includes a central core, primary and secondary coils, a case, a dielectric sheet and dielectric resin. The central core is produced from at least one magnetic steel plate. The primary and secondary coils are produced by concentrically winding primary and secondary electric wires, respectively, around the central core. The case receives the primary and secondary coils. The dielectric sheet is wound around an outer peripheral surface of the central core. An outer peripheral side axial end corner of at least one of axial end portions of the dielectric sheet is formed into a blunt smooth round shape. The dielectric resin is filled in spaces in the case.
According to another aspect of the present invention, there is provided an ignition coil, which includes primary and secondary coils, a case, a central core, a sheet and thermoset resin. The case receives the primary and secondary coils. The central core is made of a magnetic material and is positioned radially inward of the primary and secondary coils. The sheet relieves a thermal stress and is wound around the central core. At least one of axial end portions of the sheet forms a folded end portion, which is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. The folded end portion includes a plurality of generally arcuate segments, which are folded one after another on one circumferential side thereof. A first circumferential end and a second circumferential end of each generally arcuate segment are folded in such a manner that the first circumferential end of the generally arcuate segment is folded over a second circumferential end of an adjacent one of the plurality of generally arcuate segments located on a first circumferential end side of the generally arcuate segment, and the second circumferential end of the generally arcuate segment is folded beneath a first circumferential end of another adjacent one of the plurality of generally arcuate segments located on a second circumferential end side of the generally arcuate segment. The thermoset resin is filled in spaces in the case.
According to another aspect of the present invention, there is provided an ignition coil, which includes primary and secondary coils, a case, a central core, a sheet and thermoset resin. The case receives the primary and secondary coils. The central core is made of a magnetic material and is positioned radially inward of the primary and secondary coils. The sheet relieves a thermal stress and is wound around the central core. At least one of axial end portions of the sheet forms a folded end portion, which is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. The folded end portion includes a plurality of generally arcuate bottom side segments and a plurality of generally arcuate top side segments, which are alternately arranged in a circumferential direction. A first circumferential end and a second circumferential end of each generally arcuate top side segment are folded in such a manner that the first circumferential end of the generally arcuate top side segment is folded over an adjacent one of the plurality of generally arcuate bottom side segments located on a first circumferential end side of the generally arcuate top side segment, and the second circumferential end of the generally arcuate top side segment is folded over another adjacent one of the plurality of generally arcuate bottom side segments located on a second circumferential end side of the generally arcuate top side segment. The thermoset resin is filled in spaces in the case.
According to another aspect of the present invention, there is also provided a manufacturing method of an ignition coil, which includes a central core that is produced from at least one magnetic steel plate; primary and secondary coils that are produced by concentrically winding primary and secondary electric wires, respectively, around the central core; a case that receives the primary and secondary coils; a dielectric sheet that is wound around an outer peripheral surface of the central core; and dielectric resin that is filled in spaces in the case. According to the manufacturing method, the dielectric sheet is wound around the outer peripheral surface of the central core in such a manner that at least one of axial end portions of the dielectric sheet projects from a corresponding axial end surface of the central core to form an axially projecting portion. An excess region of the axially projecting portion is melted and is cut by one of a hot wire and a laser beam in such a manner that an inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape.
According to another aspect of the present invention, there is provided a manufacturing method of an ignition coil, which includes a central core that is produced from at least one magnetic steel plate; primary and secondary coils that are produced by concentrically winding primary and secondary electric wires, respectively, around the central core; a case that receives the primary and secondary coils; a dielectric sheet that is wound around an outer peripheral surface of the central core; and dielectric resin that is filled in spaces in the case. The dielectric sheet is wound around the outer peripheral surface of the central core in such a manner that at least one of axial end portions of the dielectric sheet projects from a corresponding axial end surface of the central core to form an axially projecting portion. The axially projecting portion is heated in such a manner that an inner peripheral side axial end corner and an outer peripheral side axial end corner in the axially projecting portion are respectively formed into a blunt smooth round shape.
According to another aspect of the present invention, there is provided a manufacturing method of an ignition coil, which includes: primary and secondary coils; a case that receives the primary and secondary coils; a central core that is made of a magnetic material and is positioned radially inward of the primary and secondary coils; and thermoset resin filled in spaces in the case. According to the manufacturing method, a sheet, which relieves a thermal stress, is wound around the central core. At least one of axial end portions of the sheet is folded against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core, in such a manner that a plurality of points of the axial end portion, which are arranged one after another in a circumferential direction of the axial end portion, are partially folded first, so that a folded end portion, which includes a plurality of generally arcuate segments folded one after another in the circumferential direction, is formed.
According to another aspect of the present invention, there is provided a manufacturing apparatus for manufacturing an ignition coil, which includes: primary and secondary coils; a case that receives the primary and secondary coils; a central core that is made of a magnetic material and is positioned radially inward of the primary and secondary coils; and thermoset resin filled in spaces in the case. The manufacturing apparatus includes a folding jig that folds an axially projecting portion of a sheet, which relieves a thermal stress and is wound around the central core, against a corresponding axial end surface of the central core or an axially outer end surface of a connecting member connected to the corresponding axial end surface of the central core. An axial end of the folding jig includes a recessed processing portion that engages a plurality of points of the axially projecting portion, which are arranged one after another in a circumferential direction of the axially projecting portion, to form creases in the plurality of points, respectively.